Heading into tonight's critical first presidential debate, the Romney campaign is aggressively parrying President Barack Obama's attacks on his tax reform plan while jumping on Vice President Joseph Biden's gaffe Tuesday that the middle class has been "buried" during the past four years.

Biden's statement came as he accused Mitt Romney of trying to raise taxes on the middle class to pay for bigger tax cuts for the highest earners - an attack that has been a staple of the Obama campaign ever since a Tax Policy Center study found Romney's plan would add up only if the middle class paid higher taxes overall.

"How they can justify - how they can justify raising taxes on a middle class that has been buried the last four years?" Biden said at a campaign stop. "How in Lord's name can they justify raising their taxes with these tax cuts?"

Romney running mate Paul Ryan immediately jumped on the remarks. "Vice President Biden, just today, said that the middle class, over the last four years, has been 'buried.' We agree."

And the Romney campaign quickly pounced with former New Hampshire Gov. John Sununu saying on a conference call, "Thank you, Vice President Biden. For the first time in a long, long time, you're right."

For Romney's campaign, the gaffe was a golden opportunity to try to refocus the debate on the still-struggling economy while making the case that the former Massachusetts governor has plenty of time to sway voters.

"A great deal of America has never seen Gov. Romney except in news clips," Sununu said. Romney will be able to show he has "a history of understanding the impact problems have on people" and can transfer "that empathy" and his ability to solve problems that he showed as governor of Massachusetts to the presidency.

And while the campaign used the gaffe to reiterate all of the sluggish economic statistics that Romney has repeatedly sought to pound home - the campaign also pushed back against the underlying attack that Romney's plan would raise taxes on the middle class.

The Romney campaign has disputed the Tax Policy Center analysis but has not detailed how he would pay for his proposals to cut tax rates by 20 percent and eliminate the estate tax without increasing the deficit.

Romney floated one possibility during an interview with Denver's Fox31 television Monday night - capping tax deductions at $17,000.

"As an option, you could say everybody's going to get up to a $17,000 deduction, and you could use your charitable deduction, your home mortgage deduction or others - your health care deduction, and you can fill that bucket, if you will, that $17,000 bucket that way. And higher income people might have a lower number. Or you could do it by the same method that Bowles-Simpson did it, where you could limit certain deductions, but that's the sort of thing you do with Congress."

A $17,000 cap would result in tax increases on many middle-class people who itemize their tax returns, have expensive health insurance plans, big health care bills or sizeable mortgages. It could also result in tax increases on people such as Romney, who has cut his tax bill significantly by making millions of dollars in charitable contributions.

The Romney camp also pointed to an American Enterprise Institute blog post suggesting that it is possible to cut tax rates by 20 percent across the board without shifting the overall tax burden to the middle class. The Tax Policy Center has repeatedly calculated that there is no way Romney's plan could do so, even if the highest earners lost all of their tax deductions.

The AEI blog post suggested economic growth could make up the lost revenue and argued that Romney's plan to eliminate tax increases under the health care law should not be counted as part of any analysis.

The Obama campaign continues to press the issue and hammer Romney's own finances, which include millions of dollars in overseas investments.

Obama spokeswoman Jen Psaki told reporters that Romney should have to answer questions about his overseas investments during the debate. And Psaki also highlighted a report by the Commonwealth Fund that 72 million people would be uninsured under Romney's proposals.

Psaki also dinged Romney for saying he would not deport illegal immigrants brought here as children who have bought work visas under a program Obama announced several months ago. Romney had repeatedly ducked the issue in recent months, and Psaki said Romney's position remains confusing.

"Does he still believe the Arizona law is a model for comprehensive immigration reform?" she asked.

The immigration issue has helped Obama open up a huge lead on Romney among Hispanic voters.